Capacitive sensing array having electrical isolation

ABSTRACT

A capacitive fingerprint sensor that may be formed of an array of sensing elements. Each capacitive sensing element of the array may register a voltage that varies with the capacitance of a capacitive coupling. A finger may capacitively couple to the individual capacitive sensing elements of the sensor, such that the sensor may sense a capacitance between each capacitive sensing element and the flesh of the fingerprint. The capacitance signal may be detected by sensing the change in voltage on the capacitive sensing element as the relative voltage between the finger and the sensing chip is changed. Alternately, the capacitance signal may be detected by sensing the change in charge received by the capacitive sensing elements as the relative voltage between the finger and the sensing chip is changed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 14/022,058, filed Sep. 9, 2013, entitled “Capacitive Sensing Array Having Electrical Isolation,” the contents of which are incorporated by reference as if fully disclosed herein.

TECHNICAL FIELD

Embodiments described herein relate generally to circuits that may be used to support biometric sensing, such as biometric sensors and circuits having an electrical isolation construct to electrically isolate the sensors from external components.

BACKGROUND DESCRIPTION

The following terminology is exemplary, and not intended to be limiting in any way. The text “capacitive sensing element”, and variants thereof, generally refers to one or more data elements of any kind, including information sensed with respect to individual locations. For example and without limitation, a capacitive sensing element can include data or other information with respect to a relatively small region of a fingerprint image. After reading this application, those skilled in the art would recognize that these statements of terminology would be applicable to techniques, methods, physical elements, and systems (whether currently known or otherwise), including extensions thereof inferred or inferable by those skilled in the art after reading this application.

Fingerprint sensing technology has become widespread in use and is often used to provide secure access to sensitive electronic devices and/or data. Generally, capacitive fingerprint sensors may be used to determine an image of a fingerprint through measuring capacitance through each capacitive sensing element of a capacitive sensor. The higher the capacitance, the nearer the surface of an adjacent or overlying finger to the capacitive sensing element. Thus, fingerprint ridges provide a higher capacitance in an underlying capacitive sensing element than do fingerprint valleys.

Capacitive fingerprint sensors come in at least two varieties, namely active and passive. Active capacitive sensors are often used in electronic devices to provide biometric security and identification of users.

Active capacitive sensors initially excite the epidermis of the sensed finger. Capacitance to the epidermis is measured at each capacitive sensing element. As one example, capacitance may be measured or determined by measuring a capacitive sensing element's voltage and/or charge during a low voltage phase and a high voltage phase of a modulation frequency for the capacitive sensing element array. The difference in voltages may be used to determine capacitance. One example of an active capacitive sensor is shown in FIG. 1.

As shown in FIG. 1, the active fingerprint sensor may include both capacitive sensing element array 102 on sensor chip 100 and drive ring 104. The voltage of capacitive sensing element array 102 is not directly driven or modulated, but instead drive ring 104 is modulated by drive amplifier 106. This, in turn, excites finger 108 and the voltage and/or charge at each capacitive sensing element of capacitive sensing element array 102 varies as drive ring 104 is modulated since finger's 108 voltage potential changes with the modulation of drive ring 104.

In such a sensor, the voltage that may be applied to the drive ring may be limited. Commonly, the drive ring voltage is no more than 4 volts peak-to-peak. Voltages above this may excite the finger to too high a voltage; this excessive excitation may be detected by a person as a “tingling” or uncomfortable feeling in their finger. Although the exact voltage at which one can sense the tingling varies from person to person, a 4 volt peak-to-peak voltage is generally considered as the threshold beyond which the feeling is noticeable.

Since the drive ring's voltage is restricted to avoid user perception, the thickness of any dielectric overlaying the sensor may also be limited. The thicker the dielectric between sensor pad and finger, the more attenuated the resulting capacitance and the blurrier the fingerprint image becomes. For dielectrics having a thickness or more than approximately 100 microns, the fingerprint image may become unreliable.

Another limitation arises when other parts of the user's finger or hand or body may capacitively couple through earth ground to the system, or directly to the system ground when touching other parts of the system. This capacitive coupling from the user to the system may be highly variable depending on how the user is touching the device. This parasitic coupling attenuates the voltage that the drive ring is able drive into the user's finger, and as such reduces the signal. The attenuation may be highly variable depending on how the user is touching the device.

SUMMARY

Embodiments described herein may take the form of an electronic device, comprising: a sensor chip comprising an array of individual capacitive sensing elements; an electronic component positioned nearby the sensor pad within the electronic device; and an electrical isolation wall electrically separating the sensor pad and electronic component.

Other embodiments described herein may take the form of an electronic device, comprising: a housing; an antenna at least partially within the housing; an input element separate from and adjacent the housing; a sensor positioned beneath the input element; and an electrical isolation element electrically isolating the antenna from the sensor.

Still other embodiments will be apparent to those of ordinary skill in the art upon reading the following disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a block diagram of a sample capacitive sensing array.

FIG. 2 depicts a block diagram of an embodiment of a capacitive sensing array, as described herein.

FIG. 3 depicts a sample electronic device incorporating the embodiment of a capacitive sensing array.

FIG. 4 is a schematic illustration of a fingerprint sensing system according to an embodiment.

FIG. 5 is a schematic illustration of an inductive boost circuit in accordance with an embodiment, together with a timing diagram showing example operation of the inductive boost circuit.

FIG. 6 is a schematic illustration of another fingerprint sensing system in accordance with an embodiment.

FIG. 7 is a schematic illustration of another biometric sensing system.

DETAILED DESCRIPTION

Generally, embodiments discussed herein may take the form of a capacitive sensor, such as a fingerprint sensor. The sensor may be formed from a capacitive sensing element array; each capacitive sensing element of the array may register a voltage that varies with the capacitance of a capacitive coupling. A finger may capacitively couple to the individual capacitive sensing elements of the sensor, such that the sensor may sense a capacitance between each capacitive sensing element and the flesh of the fingerprint. The capacitance signal may be detected by sensing the change in voltage on the capacitive sensing element as the relative voltage between the finger and the sensing chip is changed. Alternately, the capacitance signal may be detected by sensing the change in charge received by the capacitive sensing elements as the relative voltage between the finger and the sensing chip is changed. Portions of the finger further away from the sensor may create a lower capacitance between the sensor and finger, and thus a lower signal on underlying capacitive sensing elements. By contrast, portions of the finger closer to the sensor may create a higher capacitance between sensor and finger, and thus higher signals on underlying capacitive sensing elements. Thus, capacitive sensing elements underlying ridges of a fingerprint may register higher signals while capacitive sensing elements underlying valleys of the fingerprint may register a lower capacitance and lower signals.

The fingerprint sensor may include both a sensor pad and a drive ring. Both the sensor pad and drive ring may be placed beneath a dielectric, such that the finger does not directly touch either the sensor pad or drive ring but instead comes into contact with the interposed dielectric. In some examples, the finger may contact the drive ring.

The difference in signals at the sensor capacitive sensing elements may be used to map a fingerprint. The resolution of the sensor may vary with capacitive sensing element density, distance between the sensor pad's surface and the finger, and thickness of a dielectric covering the sensor pad. Generally, as the dielectric increases in thickness, the capacitance between the finger and sensor grows increasingly attenuated, and so the signal differences may attenuate, as well. Certain embodiments may address this attenuation by applying a higher, less variable, relative voltage change between the finger and the sensor pad, leading to a higher detectable signal at the sensor, as described in more detail herein.

By driving a higher voltage to the sensor, the capacitive coupling between the sensor and finger may compensate for the thickness of the dielectric. By increasing the voltage in this fashion, the capacitance between finger and capacitive sensing elements of the sensor pad may be increased, thereby obtaining better resolution and imaging of the fingerprint. The sensor may be driven at a higher voltage, such as 12 volts peak-to-peak, 16 volts peak-to-peak, or even more, without inducing any physical sensation in the finger or hand of a user. This may be accomplished by maintaining the drive ring's voltage at a system ground voltage, while the sensor array is modulated with the higher peak-to-peak voltage signal.

By driving a voltage to the sensor relative to system ground, while maintaining the drive ring's voltage at system ground, minimizes the issue of signal attenuation due to the highly variable capacitive coupling between the user and the system ground as a result of other fingers, hands, or body parts coming in contact with other parts of the device.

FIG. 2 depicts one sample embodiment of a fingerprint sensor. Sensor 200 may include sensor pad 202 formed by an array of individual capacitive sensing elements. Although sensor pad 202 is shown, the individual capacitive sensing elements are omitted for purposes of clarity. As previously mentioned, each capacitive sensing element may be capacitively coupled to a portion of finger 213 overlying that capacitive sensing element; the distance between the overlying finger portion and capacitive sensing element determines the capacitance between the two and thus the signal registered at the capacitive sensing element. As distance decreases, signal increases. Each capacitive sensing element is also capacitively coupled to other adjacent overlying finger portions, and as the distance increases, this leads to a blurring effect, which reduces the total signal difference between capacitive sensing elements directly under finger ridges and finger valleys.

The fingerprint sensor may also include drive ring 204. Drive ring 204 may be maintained at a system ground voltage, rather than an absolute or earth ground.

The fingerprint sensor may also include first and second drive amplifier 206 and 208, as depicted in FIG. 2. First drive amplifier 206 may drive a sensor integrated circuit that includes sensor pad 202, such as an application-specific integrated circuit (ASIC) with a modulating voltage, to induce relative voltage between sensor pad 202 and user finger 212. Second drive amplifier 208 may, provide voltage and current to the sensor integrated circuit, so that the sensor integrated circuit can operate from the relative voltage difference between first and second drive amplifiers 206 and 208. Second drive amplifier 208 may supply a modulating voltage in sync with first drive amplifier 206, such that the relative voltage between the outputs of the first and second amplifiers is approximately constant. The first and second drive amplifiers may be connected by capacitor 210, which can help amplifiers 206 and 208 keep the relative voltage between the first and second drive amplifier outputs constant.

FIG. 3 depicts electronic device 300 that may incorporate a fingerprint sensor, such as that shown in FIG. 2. Electronic device 300 may be a mobile telephone, a tablet computing device, a notebook computer, a personal digital assistant, a desktop computer, a portable media player, and the like. The sensor pad may be placed below an input mechanism, such as button 302 of electronic device 300; the button may be a part of the housing or adjacent the housing. The sensor pad may alternatively be placed beneath a portion of display screen 304 of electronic device 300, beneath sidewall 306 or other portion of the device's enclosure, and the like. Essentially, any portion of the electronic device's enclosure may house the fingerprint sensor.

In some embodiments, a fingerprint sensor chip (including both sensor pad and drive ring) may be positioned beneath the button 302. An ink layer and/or adhesive may be placed between the button's bottom surface and the sensor chip's top surface. The adhesive may bond the chip to the button, for example. One or more solder balls may affix the fingerprint sensor chip to a flex conductor. The solder balls may generally be placed near the center of the fingerprint sensor chip to reduce the likelihood of cracking due to stress.

In still other embodiments, a number of fingerprint sensors as described herein may be employed in a single electronic device. For example, the fingerprint sensors may be arranged beneath a cover glass or outer surface of an electronic device. In some embodiments, the sensors may be arranged in an array or other ordered pattern. In other embodiments, the sensors may be placed randomly or semi-randomly beneath the surface. In still other embodiments, the sensors may be placed within or beneath a display stack of an electronic device incorporating a display, such as a mobile phone, tablet computing device, computer display screen, and the like. In such embodiments, the sensor(s) may capacitively sense a fingerprint through the display and/or cover glass of the display.

Further, in such embodiments, a portion of the device enclosure or frame may serve as a ground ring as described herein. Multiple sensors may share a ground ring in such a configuration. For example, if the mobile device may capacitively sense touch, the device may activate a fingerprint sensor at, under or near the location at which a touch was sensed. In some embodiments, only fingerprint sensors corresponding to the touch location may be activated while others remain inactive. The ground ring (e.g., portion of the housing, such as sidewall 306) may be coupled to the active fingerprint sensor to obtain a fingerprint or partial fingerprint and operate as generally described herein.

As yet another option, one or more fingerprint sensors may be placed outside of the display area 304 but beneath a surface of the electronic device 300. For example, one or more sensors may be located in a chin (e.g., a region below the display 304, such as the area around button 302) or a forehead (e.g., a region above the display 304) of the electronic device 300.

In some embodiments, flex circuit may extend from the input mechanism stack-up and connect the sensor chip to a signal provider chip that may facilitate modulating the voltage and/or operating frequency of the fingerprint sensor chip. In some examples, the signal provider chip and fingerprint sensor chip may be provided as a single chip or distributed amongst a different number of chips. External components may also be used. As one example, an ASIC may be used as the signal provider chip. The ASIC may also communicate data to and from a processor of the electronic device.

FIG. 4 is a schematic illustration of a fingerprint sensing system according to an embodiment. The fingerprint sensing system 400 includes a fingerprint sensor chip 405 and a signal provider chip 410. A drive ring 412 is also shown. During operation, the sensor provided by the fingerprint sensor chip may be modulated using a higher voltage, e.g. 16V peak-to-peak, while the drive ring 412 may be held at a system ground voltage. The system ground voltage may be provided by the signal provider chip 410 and is shown as NRING in FIG. 4. The 16V peak-to-peak signal may be provided by the signal provider chip 410, and is shown as NGND in FIG. 4.

General operation of the fingerprint sensing system shown in FIG. 4 will now be discussed. It is to be noted that in alternative embodiments certain components shown in FIG. 4 may be omitted, others added, or both. The distribution of components as between external components, the fingerprint sensor chip, and the signal provider chip may also be different in other embodiments. Conceptually, a sensor pad (not shown in FIG. 4) may be integrated into or coupled to the fingerprint sensor chip 405. The capacitive sensing element array may capacitively couple to a finger of a user's hand.

The capacitive sensing element array may be modulated at a variety of frequencies. The capacitive sensing element array may measure the voltage of all capacitive sensing elements during a high voltage and again during a low voltage. That is, voltages supplied to the capacitive sensing element array and/or the sensor chip ground voltage, may generally resemble a square wave, although in alternative embodiments the voltage may take other wave shapes. A modulator (labeled “mod. block” in FIG. 4) may control the modulation or oscillation of the capacitive sensing element array and/or the sensor chip ground voltage in accordance with a clock signal, NDRV in FIG. 4. The difference between the capacitive sensing element voltages and/or charges, as measured during the high and low portions of the modulation cycle, permits the chip to determine the capacitance between each capacitive sensing element and overlying portion of the finger and thus the capacitance. From the capacitance with each capacitive sensing element, the fingerprint may be sensed, measured and reconstructed as known to those skilled in the art and generally discussed above.

The signal provider chip 410 may provide a modulated high voltage signal (e.g. 16V peak-to-peak) to the fingerprint sensor chip 405 for use in modulating the sensor. While 16V is used here as an example, in other examples different voltages may be used, including voltages greater than 4V in some examples, greater than 6V in some examples, greater than 8V in some examples, greater than 10V in some examples, greater than 12V in some examples, greater than 14V in some examples, and greater than 16V in some examples. The signal provider chip 410 may further provide a ground voltage, shown as NRING in FIG. 4 to the drive ring 412. Other voltages for NRING may be used in other examples, including 4V, less than 4V, less than 2V, or negative voltages. In some examples, the drive ring 412 may be kept at a voltage that may not be felt by a user touching the drive ring 412 but that has a sufficient difference in magnitude from the voltage provided to the sensor (e.g. NGND) to provide suitable signal-to-noise ratio for fingerprint sensing. In one example, that difference in magnitude is 16V, with the sensor voltage provided being 16V and the drive ring held at a ground voltage.

Example components of the fingerprint sensing system 400 used to form an inductive boost circuit to provide the NGND signal (which may be, e.g. 16V) will now be described. Reference will be made to transistors 421, 422, and 423, inductor 425, timing block 427, diode 442, diode 443, and level translators 430. A clock signal, e.g. NDRV of FIG. 4, may be received by the timing block 427. The clock signal may be provided to a level translator 430 prior to receipt by the timing block 427 in some examples. The level translator 430 may function to change a magnitude of the clock signal in some examples where, e.g. the timing block may operate in a different supply power domain than the fingerprint sensor chip 405. The timing block 427 may output respective versions of the clock signal (e.g. NDRV) to the gates of the transistors 421, 422, and 423. The respective versions of the clock signals may be delayed varying amounts as applied by the timing block 427. For example, the timing block may include one or more delay circuits configured to delay the clock signal (e.g. NDRV) prior to providing a delayed signal to the transistors 421, 422, and/or 423.

The transistor 423 may serve as a pass transistor. For example, the transistor 423 may turn on when a clock signal, e.g. NDRV, is provided to the timing block 427. Accordingly, the signal provided by the timing block to a gate of the transistor 423 may be indicative of the presence or absence of a clock signal (e.g. NDRV). In this manner, the transistor 423 may be turned on when the fingerprint sensor chip 405 is active and may be turned off when the fingerprint sensor chip 405 is inactive. The signal provided by the timing block 427 to the gate of the transistor 423 may be provided through an optional gate driver circuit as shown. When the transistor 423 is turned on, a supply voltage (e.g. 1.8 V in some examples) may be provided to the inductor 425, to allow the inductor 425 to store energy. The inductor may charge capacitor 432 to a boosted voltage, e.g. a voltage greater than the power supply voltage. In some examples, the inductor provides a boosted voltage of 16 V. In some embodiments, capacitor 432 can be implemented as a physical component of the system, in some embodiments the capacitor 432 may be implemented as the parasitic capacitance of the fingerprint sensor 405 to the environment and the system ground, and combinations of these may be used in some embodiments.

The transistor 421 may function to chop a voltage provided by the inductor 425 at a frequency expected by the fingerprint sensor chip 405, such as 2 MHz in some examples. In some examples, the transistor 421 may chop the voltage at a same frequency as the clock signal, e.g. NDRV. When the transistor 421 is off, the voltage NGND may be a boosted power supply voltage developed by the inductor 425, e.g. 16V. When the transistor 421 is on, the current flowing in the inductor 425 may find a path to ground, and the voltage NGND may transition to ground. Accordingly, the transistor 421 may be switched at a particular frequency to achieve a shaped 16V peak-to-peak waveform. Other peak voltages may also be used. In some examples, the switched frequency is the same as the frequency of the clock signal, e.g. NDRV. Accordingly, the timing block 427 may delay the clock signal, e.g. NDRV, a particular amount and provide a delayed clock signal (e.g. delayed NDRV) to a gate of the transistor 421. The amount of delay may be related to the desired NGND voltage. In one example, the delay is 25 ns when a 2 MHz clock is used. In this manner, the NGND signal provided at the node labeled NGND may be a boosted (e.g. 16 V) square wave signal at a frequency of 2 MHz. Other frequencies and voltages may be used.

In some examples, it may be disadvantageous to have the inductor 425 coupled directly to the node providing NGND. For example, the inductor 425 may then be coupled to parasitic capacitances, shown in FIG. 4 as C_(para) 432. The parasitic capacitances may represent, for example, capacitance between the fingerprint sensor chip 405 and ground, and may include capacitances through a body of the user touching the fingerprint sensor. When the inductor 425 is coupled to the parasitic capacitance 432, oscillation of the boosted voltage (e.g. NGND) signal may occur. To minimize or remove oscillations, the transistor 422 may be provided. When turned on, the transistor 422 may advantageously divert the current from the inductor 425 to damp oscillations which otherwise may occur. Accordingly, the timing block 427 may provide a version, which may be a delayed version, of the clock signal (e.g. NDRV) to the gate of the transistor 422. The clock signal (e.g. NDRV) may be delayed by the timing block 427 an amount sufficient such that when the transistor 422 turns on, the voltage NGND has already reached the target voltage (e.g. 16V).

In this manner, an inductive boost circuit may be used to provide a boosted voltage (e.g. 16 V) at the node labeled NGND in FIG. 4 (after the diode 443). The boosted voltage may be provided to the fingerprint sensor chip 405 for use in modulating the sensor. The boosted voltage itself may be provided as a modulated signal having a frequency, which may be the same as a frequency of the system clock signal (e.g. NDRV).

The low dropout regulator (LDO) 450 may provide a constant supply voltage to the fingerprint sensor chip (e.g. 1.8V). This voltage is provided respectively to the node NGND which can be at the system ground voltage or the boosted voltage.

The four communication lines labeled SCLK, MOSI, MISO and INT generally permit data to be transferred to and from the fingerprint sensor chip. The signals may be provided through the level translator 430. In the fingerprint sensor chip 405, the communication lines are referenced to the sensor ground NGND that is different from the system ground. The level translator may allow for communication to happen between the NGND referenced fingerprint sensor chip and the system ground (GND) referenced device processor. The communication may happen synchronously or asynchronously with the modulation signal NDRV. During the communication, the node NGND can be at the system ground voltage or at the boosted voltage.

SCLK is the serial communication clock and may be used for timing operations.

The MISO line may be used for master/slave transmission of data from a slave to a master. In the present embodiment, the fingerprint sensor chip may be the slave, while a processor associated with the electronic device incorporating the sensor may be the master. Likewise, the MOSI line may be used for sending data from the master (e.g., device processor) to the slave (e.g., fingerprint sensor chip).

The INT line may be used for interrupts. The fingerprint sensor chip may use the INT line to provide a signal indicating to the device processor that the fingerprint sensor chip has completed a task or that the fingerprint sensor chip has some fingerprint data ready to be transferred.

During an idle state, the fingerprint sensor chip 405 may operate in a relatively low-power state. When the chip detects a finger positioned above the capacitive sensing element array, it may initiate a scanning operation. The chip may detect a nearby finger by measuring a capacitance change of one or more capacitive sensing elements of the array relative to its ground, as one example. This may cause the chip to begin a scanning operation. The scanning operation generally may include reading the voltage or charge of one or more of the capacitive sensing elements during a low-voltage state of the array or sensor chip ground, driving the array or sensor chip ground to a high voltage and again reading the voltage or charge of one or more of the capacitive sensing elements. The scanning operation may alternately include reading the voltage or charge of one or more of the capacitive sensing elements during a high-voltage state of the array or sensor chip ground, driving the capacitive sensing element array or sensor chip ground to a low voltage and again reading the voltage or charge of one or more of the capacitive sensing elements. The capacitive sensing elements may be modulated at a relatively high frequency between the low and high voltage states by the modulator. In some embodiments, the chip may operate at a two megahertz frequency, although other embodiments may employ higher or lower frequencies.

It should be appreciated that the drive ring may not be modulated with the capacitive sensing element array. Rather, the drive ring's potential may remain constant at system ground or very near to system ground. This may help prevent or reduce excitation and/or modulation of the finger, which in turn permits the use of higher voltages to drive the capacitive sensing element array.

Further, in embodiments where the fingerprint sensor is incorporated into a hand-held device such as the mobile device of FIG. 3, other fingers of the user's hand may be in contact with a metal portion, or other conductive portion, of the enclosure. The enclosure generally is also at system ground, so contact between conductive elements on the enclosure and the user's hand facilitate driving or maintaining the voltage of the sensed finger to system ground. The “sensed finger” is the finger overlaying the sensor pad, or the finger being imaged by the fingerprint sensor. Maintaining the sensed finger at or near system ground may enhance resolution of the fingerprint image and assist in preventing modulation of the finger.

FIG. 5 is a schematic illustration of an inductive boost circuit in accordance with an embodiment, together with a timing diagram showing example operation of the inductive boost circuit. The inductive boost circuit 500 is the same used in the fingerprint sensing system 400 of FIG. 4, and like components are labeled with like reference numbers, the details of which are not repeated here for brevity. Operation of the inductive boost circuit 500 will be described with reference to the timing diagram shown in FIG. 5 for added clarity. As discussed above, the transistor 423 may function as a pass transistor that may be on whenever an active clock signal (e.g. NDRV) is present. Accordingly, for the purposes of discussion of FIG. 5, the transistor 423 is assumed ON during operations described with reference to FIG. 5. Similarly, the diode 442 may be used only when transistor 423 turns off following a fingerprint scan. Accordingly, the diode 442 is assumed OFF during described operations with reference to FIG. 5.

At time t₀, the transistor 421 may turn off (e.g. the gate voltage shown in the trace related to transistor 421 of FIG. 5 may transition low, turning the transistor 421 off). When the transistor 421 is turned off, the inductor current I_(L) decreases as the voltage at node NGND begins to rise to the boosted voltage (e.g. 16V) as shown in the timing diagram traces labeled I_(L) and NGND. The voltage of NGND may peak at time t₁, as shown, where t₁ may represent an inductor voltage buildup time following t₀. At time t₁, then, the diode 443 may turn off, not allowing current to flow through the diode 443 to capacitor 432, and thus maintaining the voltage on NGND.

At time t₂, the transistor 422 may turn on (e.g. the gate voltage shown in the trace related to transistor 422 of FIG. 5 may transition high, turning the transistor 422 on). This may allow damping through the transistor 422 to prevent or reduce oscillations in NGND. At time t₃, the transistor 421 may be turned on (e.g. the gate voltage shown in the trance related to transistor 421 of FIG. 5 may transition high, turning the transistor 421 on). When the transistor 421 is turned on, the voltage NGND may be returned to ground as the capacitor 432 is discharged through the transistor 421. Accordingly, the NGND signal falls back to ground starting at time t₃.

At time t₄ the transistor 422 is turned off (e.g. the gate voltage shown in the trace related to transistor 422 of FIG. 5 may transition low, turning the transistor 422 off). The diode 443 may accordingly turn on, allowing current to flow, but since the transistor 421 remains on, the current may not charge the node NGND until the transistor 421 is again turned off.

FIG. 6 is a schematic illustration of another fingerprint sensing system in accordance with an embodiment. The fingerprint sensing system 600 includes a fingerprint sensor chip 605, and a signal provider chip 610, which may be implemented using the fingerprint sensor chip 405 and signal provider chip 410 of FIG. 4, respectively. Drive ring 612 and fingerprint receiving surface 614 are also shown. External components inductor 625 and diodes 642 and 643 are shown which may be used by the signal provider chip 610 in providing a boosted voltage (e.g. NGND) during fingerprint scans. The inductor 625 and diodes 642 and 643 may be implemented using the inductor 425 and diodes 442 and 443 of FIG. 4, and the signal provider chip may include remaining components of the inductive boost circuit 500 of FIG. 5, for example. In FIG. 6, attention is drawn toward the power domains that may be used for each of the respective chips and result from the operation of the booster circuit in the signal provider chip 610.

For example, two domains are shown GND-referenced domain 650 and NGND-referenced domain 652. In the GND-referenced domain, operations may occur using a system ground (GND). In the NGND-referenced domain, operations may occur using a boosted ground (NGND). The boosted ground signal (e.g. NGND) may only be boosted at particular times, such as during a scan. Moreover, as described above, the boosted ground signal (e.g. NGND) may be modulated such that it has a frequency, which frequency may equal a frequency of a clock signal used by the system (e.g. NDRV). By raising the ground potential used by the fingerprint sensor chip 605, signal-to-noise ratio of fingerprint scans may be improved without impact or with minimal impact to a user experience (e.g. tingling of the finger may not be induced since the boosted voltage is not provided to the drive ring 612, and therefore a user's finger). Furthermore, the fingerprint receiving surface 614 may be made of an insulating material preventing the finger from being in contact with the sensor chip 605 and affected by the boosted voltage. Accordingly inputs to the fingerprint sensor chip 605 may be referenced against NGND, and not system ground.

Certain embodiments described herein may be placed in close proximity to other electrical components and/or circuits, especially when incorporated into electronic devices that are relatively compact or where space is otherwise an issue. For example, in electronic devices incorporating an antenna, the fingerprint sensor may be placed adjacent or near the antenna. Referring again to FIG. 2, in the depicted embodiment the sensor may be located beneath the button. Further, an antenna or (antennas) may be incorporated into the sidewalls of the electronic device, within the electronic device, at least partially within the electronic device, as a portion of the housing, and so forth. Thus, the distance between the sensor and the antenna may be relatively small, insofar as the button is located close to the sidewall.

In some embodiments, the antenna may be a cellular antenna (such as a CDMA, GSM, and/or UMTS antenna, as well as others), a Wi-Fi antenna, a GPS antenna, a Bluetooth antenna, and so on. In certain embodiments, there may be multiple antennas that provide some or all of the foregoing functions. In some embodiments, a single antenna may provide multiple types of functionality.

The close proximity between the sensor and antenna may cause operational issues for the sensor and, in some cases, for the antenna. For example, high frequency transient signals generated by the antenna during operation may be detected by the biometric sensor and negatively impact the sensor's operation. Likewise, parasitic capacitances, transient signals and the like stemming from operation of the biometric sensor may be detected by the antenna and erroneously processed as an input signal, rather than being discarded as noise. Similarly, such noise may destructively interfere with actual data signals.

It should be appreciated that noise may be generated by a variety of electrical components beyond the aforementioned antenna; such noise maybe negatively impact operation of the sensor. The converse may also be true; operation of the biometric sensor may be detected by the antenna or other electrical component and interfere with normal operations of such components.

Thus, it may be beneficial to electrically isolate the sensor from other electrical components in an embodiment.

FIG. 7, for example, depicts one sample electronic circuit that may shield a sensor from noise generated by a nearby electronic circuit or device, such as the aforementioned antenna. This circuit is similar in some respects to the circuit described with respect to FIGS. 4 and 6, but does have some specific differences in implementation.

In the embodiment shown in FIG. 7, a controller chip 700 may generally control operations of a sensor, shown as a sensor chip 702. The controller chip may be a voltage level shifter, providing a number of different voltages to the sensor chip 702 to control or facilitate the sensor chip's operation. For example, the controller chip may supply either a three volt or a 1.8 volt signal to the sensor chip, although it should be appreciated that the exact voltages (and number of voltage signals) provided by the controller chip 700 may vary from embodiment to embodiment.

The sensor chip 702 may be a single chip or may be a set of related electronic circuitry, but is shown as a single chip for convenience of illustration. The same is true for the controller chip 700. The circuitry and electrical elements between the controller chip 700 and sensor chip 702 will now be described.

The controller chip 700 outputs a three volt signal to a common node 744 serving as the base input of diode 706. Current may flow through the diode 706 during normal (e.g., forward-biased operation).

In addition, a capacitor 704 is connected between the common node 744 and ground. Capacitor 704 may charge while current flows from the three volt output of the controller chip and the voltage at the common node 744 is three volts. Generally the capacitor is a high-speed charging capacitor to permit it to quickly charge to capacity.

A second capacitor 708 is directly electrically connected to a second side of the diode 706. The second capacitor 708 provides an electrical path to ground, and the ground of the controller chip 700. The dashed line that starts at the first capacitor 704 and extends through the diode 706, the second capacitor 708, and to the ground of the controller chip 700 illustrates the current flow of the diode reverse recovery current. This provides a path to drain the mobile charge from the diode junction and permit the diode to again block current flow after the mobile charge is drained to ground. The same path may be used to sink any dynamic current caused by the diode 706 turning off.

In this manner, the capacitor 708 may supply current to the sensor chip 702 when the three volt output of the controller chip 700 is blocked by operation of the diode 706, which typically occurs during operation of the sensor chip. Thus, so long as the period of time during which the controller chip does not supply voltage to the sensor chip is sufficiently short, the capacitor 708 may act as a battery to drive operation of the sensor chip 702.

In addition, the second capacitor 708 may operate to filter or sink alternating current noise and/or transients. The second capacitor 708 may decouple the output of diode 706 (and, ultimately, the input of the sensor chip 702) from high-frequency alternating current signal components, for example those above about 100 MHz. This may facilitate providing a steady DC input to the sensor chip 702.

Also directly electrically connected to the second end of the diode 706 (and thus the second capacitor 708) is an inductor 710. The inductor 710, along with inductors 718 and 724 and resistor 722, form an isolation wall designed to mitigate noise and other external signals (and, in particular, high-frequency signals) from reaching the sensor chip 702. High-frequency signals may include, for example, signals in the operating range of the antenna or in the operating range of another, nearby electronic component.

The inductors generally do not attenuate signals having a frequency below a cutoff value, but do not permit signals having frequencies above the cutoff value to pass therethrough. In this fashion, the inductors 710, 718, 724 and resistor 722 act as a series of low-pass filters, shielding the sensor chip 702 from high-frequency noise originating on the left-hand side of the circuit, when viewed with respect to the layout shown in FIG. 7. In some embodiments, the cutoff value is approximately 100 MHz, although this value may vary from embodiment to embodiment and may be chosen in light of the operating and/or signaling frequencies of nearby electronic circuits and components.

The end of the first inductor 710 that is opposite the end directly connected to the diode 706 may be directly electrically connected to a three volt input to the sensor chip, as well as to a third capacitor 712. The third capacitor 712 acts as a local decoupling capacitor for the three volt power supply to the sensor chip 702. That is, the third capacitor 712 may shunt alternating current signals and/or voltage spiked from the supply (e.g., the output of the first inductor 710) to a ground, rather than permitting those signals/transients to be passed to a voltage input of the sensor chip 702.

A fourth capacitor 726 may also serve as a decoupling capacitor and may be connected between the node 744 and ground, as shown in FIG. 7.

The controller chip 700 may also have a 1.8 volt input, labeled “N1V8” on FIG. 7. This output may be connected to a first side of a second inductor 718. The second inductor 718, much like the first inductor 710, may operate as a low pass filter to remove alternating current signal components from the 1.8 volt output of the controller chip 700. Accordingly, the 1.8 volt input to the sensor chip 702 may be a pure or near-pure direct current signal. Further, it should be appreciated that either the controller chip 700 or the sensor chip 702 may be used to source or sink any current described herein.

Fifth and sixth capacitors 720, 732 may be connected to a second side of the second inductor 718. The fifth capacitor 720 may be a decoupling capacitor to filter AC noise and/or transients from reaching the sensor chip, as described generally above. The sixth capacitor 732 may be a charge capacitor that stores a voltage. This voltage may be provided as an input to the sensor chip 702 to maintain the voltage level of the sensor chip during operation of the chip. The capacitor may supply voltage to a low-dropout regulator of the sensor chip, for example. The opposite sides of both capacitors 720, 732 may be connected to a second side of a third inductor 724, described below, as well as a ground of the sensor chip 702.

The controller chip 700 may also provide a clock signal (labeled “SCLK”) to the sensor chip 702. As with other electrical signal lines running from the controller chip 700 to the sensor chip 702, a high frequency filter may be placed in the signal line between the controller chip and sensor chip in order to filter high frequency noise generated by the antenna. Here, a resistor 722 is used rather than an inductor. The resistor may have a lower filtering frequency than the inductors. That is, the cutoff frequency for the resistor may be lower than the cutoff frequency for the inductors. This may be useful in the clock signal line as the clock signal may switch between high and low outputs in order to generate the clock signal. Generally, the switching frequency of the clock signal is less than the cutoff frequency of the resistor, thereby permitting the pulsed clock signal to pass through the resistor and be received by the sensor chip 702.

The ground of the controller chip 700 may be electrically connected to the ground of the sensor chip 702 through a third inductor 724. As with the other inductors, the third inductor 724 may function as a low pass filter to remove AC signals and/or transients. Accordingly, each of the signal paths between the two chips 700, 702 may have a filter thereon. The combination of the filtering inductors 710, 718, 724 and filtering resistor 722 may serve as an electrical wall to isolate the sensor chip 702 from signals produced by an adjacent antenna or other electrical component. Collectively, these elements form an electrical isolation wall. It should be appreciated that this electrical isolation works both ways. That is, the isolation wall may also electrically isolate external electrical component (such as an antenna) from noise or stray signals produced by the sensor chip 702.

In addition to the foregoing circuitry disposed between the controller chip 700 and sensor chip 702, multiple other electrical components may be electrically connected to one or more inputs or outputs of either the controller chip or sensor chip. For example, a first input decoupling capacitor 728 may be connected to the 1.8 voltage input of the controller chip 700 and to the 1.8 volt power supply. Likewise, second and third input decoupling capacitors 734, 736 may be coupled electrically to the 16 volt input of the controller chip 700 and also to the 16 volt power supply. The second sides of each of these input decoupling capacitors 728, 734, 736 may be connected to ground.

Similarly, first and second analog input decoupling capacitors 740, 742 may be connected to the analog voltage input of the sensor chip, and also to the analog voltage source. The second side of both of these capacitors may be connected to the ground of the sensor chip 702.

Another inductor 738 may be connected between a passive device and the ground of the controller chip. This inductor may likewise function to isolate unwanted signal components.

Generally, placement, layout and/or relative positioning of particular elements of the circuitry shown in, and described with respect to, FIG. 7 may facilitate the efficient operation of the circuitry. For example, it may be desirable to position capacitors 704 and 708 in close physical proximity to the diode 706. Such positioning may minimize or reduce the inductance of the current loop shown by the dashed line of FIG. 7, which in turn may enhance operation of the isolation components. Generally, the physically closer the capacitors 704, 708 are to the diode 706, the lower the current loop inductance becomes. Thus, with respect to a layout of the circuit in silicon, the relative distances of these components may be reduced in order to enhance the circuit operation.

Likewise, it may be desirable to ensure that the isolation inductors 710 and 724 are positioned outside of, and away from the current loop indicated by the dashed line in FIG. 7. The isolation inductors may be positioned at any point between the current loop and the sensor chip 702, although it may be desirable to position the inductors 710, 718, 724 relatively near to one another to ensure consistent filtering of noise on the various signal lines for the sensor chip and operation of the sensor chip.

In a similar fashion, placing the ground connection of capacitor 708 near to (and optionally as near as the silicon layout permits) to the GND2 pin of the controller chip 700 may likewise reduce or minimize the current loop inductance. Reducing the current loop inductance may likewise reduce noise on the N1V8 and 3.0V signal lines between the controller chip 700 and sensor chip 702.

Generally, the foregoing layout and positioning parameters of the various electrical elements may not only reduce or minimize loop inductance, but may also reduce or minimize mounting inductance for the decoupling capacitors and between the capacitor 708 and GND2 pin of the controller chip 700. Reduction of inductance in the sample circuit may enhance operation of the electrical isolation wall.

Although embodiments have been described herein with respect to particular configurations and sequences of operations, it should be understood that alternative embodiments may add, omit, or change elements, operations and the like. Accordingly, the embodiments disclosed herein are meant to be examples and not limitations. 

We claim:
 1. An electronic device, comprising: a display comprising an active display area; a sensor chip positioned behind the active display area and comprising an array of individual capacitive sensing elements; an electrical isolation wall comprising a first filter in series with a voltage supply of the sensor chip and a second filter in series with a ground reference of the sensor chip, the electrical isolation wall configured to filter high frequency signals from interfering with the sensor chip.
 2. The electronic device of claim 1, wherein the first filter is an inductor.
 3. The electronic device of claim 2, wherein: the inductor is a first inductor; and the second filter comprises a second inductor.
 4. The electronic device of claim 1, further comprising a cover glass positioned over the display.
 5. The electronic device of claim 4, further comprising: an enclosure housing the display, the sensor chip, and the electrical isolation wall, wherein the cover glass defines a portion of an exterior surface of the enclosure.
 6. The electronic device of claim 5, wherein the sensor chip is a fingerprint sensor configured to capacitively sense a fingerprint through the active display area.
 7. The electronic device of claim 6, wherein the fingerprint sensor is additionally configured to capacitively sense the fingerprint through the cover glass.
 8. The electronic device of claim 5, wherein the sensor chip is a fingerprint sensor configured to capacitively sense a fingerprint applied to a back surface of the enclosure opposite the cover glass.
 9. The electronic device of claim 1, wherein the display comprises a touch sensor.
 10. The electronic device of claim 9, wherein the sensor chip is activated in response to the touch sensor detecting a touch input at or near a location of the active display area above the sensor chip.
 11. A portable electronic device, comprising: a housing; a display within the housing; a capacitive sensor positioned within the housing below the display; and an electrical isolation wall, comprising: a first low pass filter in series with a voltage supply of the capacitive sensor; and a second low pass filter in series with a ground reference of the capacitive sensor; wherein the first low pass filter and the second low pass filter reduce high frequency noise, thereby limiting interference with a sensing operation of the capacitive sensor.
 12. The portable electronic device of claim 11, further comprising a wireless communications antenna at least partially within the housing.
 13. The portable electronic device of claim 12, wherein the high frequency noise is generated, at least in part, by the wireless communications antenna.
 14. The portable electronic device of claim 13, wherein the wireless communications antenna is one or more of: a cellular antenna; a Wi-Fi antenna; a GPS antenna; and a Bluetooth antenna.
 15. The portable electronic device of claim 11, wherein the display comprises an active display area and the capacitive sensor is positioned behind the active display area of the display.
 16. A method for operating a capacitive sensor in an electronic device comprising a wireless communications antenna and a display comprising a touch input sensor, the method comprising: isolating the capacitive sensor from the wireless communications antenna by: providing a supply voltage through a first low pass filter to the capacitive sensor; providing a ground reference through a second low pass filter to the capacitive sensor; and detecting, by the touch input sensor, a touch input to an active display area of the display, the active display area disposed above the capacitive sensor; activating the capacitive sensor in response to the detected touch input; and obtaining, with the capacitive sensor, an image of a fingerprint providing the detected touch input.
 17. The method of claim 16, wherein obtaining the image of the fingerprint comprises initiating a scanning operation in which a voltage or charge measurement is taken from one or more capacitive sensing elements of the capacitive sensor.
 18. The method of claim 16, wherein the first low pass filter and the second low pass filter each comprise inductors in series with the capacitive sensor.
 19. The portable electronic device of claim 11, wherein the capacitive sensor is a fingerprint sensor configured to capacitively sense a fingerprint through the display.
 20. The portable electronic device of claim 15, wherein: the display comprises a touch sensor; and the capacitive sensor is activated in response to the touch sensor detecting a touch input at or near a location of the active display area. 